1. Field
The present invention relates to a light source unit that emits light and a liquid crystal display device using the same.
2. Description of the Related Art
A liquid crystal display device includes a liquid crystal panel, a circuit board, a light source unit, etc. Light source units for applying light to the liquid crystal panel are classified into the side light type (also called the edge light type) and the direct type. In the light source unit of the side light type, a light source is disposed adjacent to a side surface of a case. In the light source unit of the direct type, a light source is disposed on the bottom surface of a case so as to be opposed to the liquid crystal panel. Some of the light source units of the side light type employ a light guide plate for guiding the light emitted from the light source to the opening of the case. In the light source unit using the light guide plate, the light emitted from a side light unit that incorporates a linear light source such as a cold cathode tube or point light sources such as a light emitting diode (LED) is reflected within the light guide plate and diffused by a diffusion pattern provided in the light guide plate, whereby the light can be taken out from the opening in a planar manner.
In general, to increase the luminance of the display screen, in the light source units using point light sources such as light emitting diodes as light emitting elements, the arrangement density of light emitting elements is increased by increasing their number or increasing the current supplied to each point light source. In either case, the temperature of portions around the point light sources is increased by heat that is generated by the individual point light sources which are emitting light. There have been proposed a light source unit including a light source board mounted with point light sources and provided with a wiring pattern for supplying power to the individual point light sources, and a heat radiating means (For example, JP-A-2002-229022, JP-A-2003-076287). Capable of increasing the heat radiation, these light source units allow increase in the arrangement density of point light sources or the current supplied to each point light source.
To output uniform light the light emitted from the point light sources through the light guide plate in a planar manner, the diffusion pattern provided in the light guide plate is formed in an optimum manner. Therefore, to prevent positional deviation between the light source board and the diffusion pattern, it is necessary to fix the light source board which is mounted with the point light sources using a positioning jig, screws, an adhesive (e.g., double-sided adhesive tapes), or the like.
In the above-described light source units, the arrangement density of light sources or the current supplied to each light source is increased by providing the heat radiation member for radiating heat generated by the light sources. However, the heat radiation member makes it difficult to reduce the width of a frame or the thickness of the light source unit, and the number of components increases.
Where the light source board mounted with the light sources is fixed with screws, spaces for screw heads are needed and the frame thickness needs to be increased for attachment of the screws. This makes it difficult to reduce the width of a frame or the thickness of the light source unit, and the number of components increases.
Where the light source board which is mounted with the light sources is fixed with an adhesive (e.g., by using double-sided adhesive tapes), the adhesive may peel off as time elapses due to influence of heat etc., which is not able to provide sufficient reliability.
Where the light guide plate is used, if the position of the light source board which is mounted with the light sources is deviated, it no longer conforms to a prescribed diffusion pattern, which results uniform distributed light in a planar manner cannot be output.